This application claims the benefit of international application number PCT/NZ00/00076 filed May 15, 2000. The international application was published under PCT Article 21(2) in the English language.
This invention relates to a method and apparatus for a position estimation system and in particular, but not exclusively to a method and apparatus for an indoor position estimation system utilising the multipath-free component of signals received from an object.
There are a number of systems available which are used to identify the position of an object or track the movements of an object. These systems may be divided into indoor and outdoor systems.
In outdoor position estimation systems, probably the most widespread is the NAVSTAR Global Positioning System (GPS). This system operates by providing a plurality of satellites transmitting reference signals from which the GPS receiver must calculate its position. This requires the device to include relatively expensive timing and calculating devices which add to the size and expense of the device. Furthermore, the magnitude of error permissible in outdoor systems is typically much greater than the requirements for an indoor system. Although the accuracy of outdoor systems may be increased using methods such as relative and differential GPS, this adds additional expense and space requirements to the system. Other outdoor position determination techniques include LORAN and radar systems. However, these also require the use of relatively expensive and bulky equipment.
In indoor applications, control over the magnitude of errors must typically be much more stringent due in part to the reduced total area in which a product may be located and the high number of objects which may be located in any particular area. Also, the smaller size of the objects result in limitations in the power of signals which may be transmitted from the object, causing systems relying on such transmissions susceptible to errors due to noise.
One known method for determining the position of a object is to detect the time of arrival of signals transmitted by a transmitter located on the object or estimating the round trip time of a signal from a polling base station to the object and back again, see U.S. Pat. No. 5,526,357. The position of the object is then determined by spatial calculations.
A problem in wireless systems is the occurrence of interfering signals. These interfering signals typically originate from a number of sources including background noise, separate transmission sources and multipath effects. Background noise and separate transmission sources are often unavoidable, but can be filtered out or accommodated in system design in some circumstances. Interference due to multipath effects can be more problematic, especially where the timing and/or phase information contained in the signal is important.
It is an object of the present invention to overcome or at least alleviate problems in methods and apparatus for tracking systems at present, or at least to provide the public with a useful choice.
According to one aspect of the present invention, there is provided position estimation system for locating an object in a given volume, the system including:
a signal transmitter suitable for use with the object, the signal transmitter adapted to transmit an identifiable signal;
a plurality of receivers suitable for receiving signals from the signal transmitter, the receivers positioned at predetermined or determinable positions in relation to the given volume;
signal detector adapted to determine the difference in the time of arrival of the signal between at least two receivers; and
computation means to compute the position of the object in the given volume from said difference in the time of arrival;
wherein the difference in the time of arrival of the signal is determined by comparing the arrival time of substantially corresponding trigger portions, said trigger portions being at least a portion of the substantially multipath free component of the signal as received at each receiver.
Preferably, the system includes computation means adapted to determine a best fit model to the trigger portion of each received signal, and compute the time difference of arrival from the time delay between the best fit models.
Preferably, the tracking system further includes a storage means adapted to store a predetermined model of the object and/or given volume, the computation means adapted to determine a pre-estimate of the onset time of the first multipath component from the predetermined model.
According to another aspect of the present invention, there is provided a method of tracking an object in a given volume, the method including:
transmitting an identifiable signal from the object to be tracked;
receiving the transmitted signal at a plurality of receivers positioned at predetermined or determinable positions;
computing the difference in the time of arrival of the signal between at least one pair of receivers based on the arrival time of at least a portion of the substantially multipath free component of the signal as received at each receiver; and
computing the position of the object in the given volume from said difference in the time of arrival.
Preferably, the method further includes computing a best fit model to the substantially multipath free component of the signal, wherein the time difference of arrival of the received signals is determined by the time delay between the best fit models.
Preferably, the method further includes the step of determining a pre-estimate of the onset time of the first multipath signal from a predetermined model of the object and/or the given volume.